Due to a rapid proliferation of wireless devices represented by mobile phone terminals, demands for high-frequency filters have been increasing at a rapid pace. In particular, there are strong demands for small-size and highly precipitous acoustic wave filters.
As the sophistication of wireless systems has advanced at a rapid pace in recent years, specific demands for high-frequency filters have become highly complex. For example, it is preferable that a transmission filter and a reception filter included in a duplexer can achieve a low loss in a passband and high suppression in the opposite band (the band of a reception filer with respect to the band of a transmission filter, and vice versa).
Generally, filters incorporated in a mobile phone terminal or the like include, in many cases, a plurality of resonators that are connected to each other so as to ensure a broad passband. For example, Japanese Laid-open Patent Publication No. 2004-15397 discloses an example of a ladder filer.
However, with the above conventional configuration, when the number of stages in the ladder filter is increased to increase the number of attenuation poles in suppression bands, a loss in the passband also increases.
Further, with the configuration of connecting inductors to parallel resonators, it is difficult to place attenuation poles in frequency bands where suppression is most needed.